Attenuated total reflection measurement (also referred to as ATR) method is a method for measuring absorption property of a sample by bringing a crystal (prism) having larger refractive index than the sample into contact with the sample, and making a measurement light incident at an incident angle that total reflection occurs at its boundary surface to detect a spectrum of a total reflection light. It is suitable for analyzing a substance having high light absorption, such as a solute in an aqueous solution that is difficult to perform spectrum measurement in an infrared region. Particularly, when a microscopic part of a sample is analyzed, an infrared microspectroscopic device having a reflected light system is used.
Patent Literature 1 discloses an infrared microspectroscopic device capable of total reflection measurement. This microscopic device comprises a Cassegrain type objective mirror which holds an ATR crystal. The objective mirror comprises a main mirror comprised of a concave mirror having a hole at its center and a sub-mirror comprised of a convex mirror having a diameter smaller than the main mirror, and the main mirror and the sub-mirror are disposed oppositely with their central axes aligned with each other. Further, the ATR crystal is mounted to an end part of a holding frame and is disposed to coincide with a focusing position of the Cassegrain mirror.
Usually, only a part in contact with the sample can be subjected to total reflection measurement. However, in recent years, a method for subjecting a certain region to mapping measurement of which the ATR crystal and the sample are not relatively moved and kept in a contact state (so-called smart mapping) has become widespread. For example, like a total reflection measurement device described in Patent Literature 2, a light from a contact surface of a sample and an ATR crystal is focused by a Cassegrain mirror and is directed to a detection-side scan mirror. When the orientation of a reflection surface of the detection-side scan mirror is fixed in a certain direction, only the light from a specific part of the sample surface corresponding to the orientation of the reflection surface passes through an aperture, and the light from other parts are blocked. That is, a conjugate position on the contact surface with respect to the aperture can be changed by changing the orientation of the detection-side scan mirror. Thus, a measurement position on the contact surface can be scanned without moving the sample, so that mapping measurement can be performed to a specific region in the contact state.